A dominant transforming gene present in late passages (330) of a human teratocarcinoma cell line (PA-1) was isolated as a biologically active molecular clone and is a new isolate of the human N-ras locus. Its transforming activity is due to a single GYieldsA point mutation at the codon for amino acid 12, which changes the codon for glycine to an aspartic acid residue. DNA from the PA-1 cell line at early passages (36) does not yield foci in DNA transfection assays and the early passage cells are much less tumorigenic in nude mice. These results, therefore, correlate the presence of an activated N-ras locus with the enhanced tumorigenicity of a cell line. The activated N-ras gene was, therefore, either selected from a small population of the original, metastatic tumor cells or arose by mutation during passage in culture.